The ink jet recording method is a printing method in which droplets of an ink are allowed to fly and deposited on a recording medium such as paper to perform printing. As the ink, there has been generally used one in which various water-soluble dyes are dissolved in water or a mixture of water and a water-soluble organic solvent. It has been generally pointed out that recorded matter obtained by using such a water-soluble dye-containing ink is poor in water resistance and light resistance.
In contrast, an ink obtained by dispersing a pigment in an aqueous medium is excellent in water resistance and light resistance. For example, there has been proposed an aqueous pigment ink obtained by dispersing a pigment by using a surfactant or a polymeric dispersing agent. Further, there has been disclosed a surface-treated pigment ink in which only a pigment can be dispersed in an aqueous solvent even when a dispersing agent such as the surfactant or polymeric dispersing agent as described above does not exist (for example patent document 1).
Generally, however, an ink using a pigment has the problems of insufficient fixing properties of the ink to recorded matter and poor rubbing resistance. That is to say, in the recorded matter obtained by using the ink in which the pigment is used as a coloring agent, the coloring agent component is liable to remain near a surface of the recorded matter. Accordingly, when the coloring agent is not sufficiently fixed to the surface of the recorded matter, there are problems in rubbing resistance, such that an image becomes dirty on rubbing with the finger or marking with a marker pen. In particular, when the pigment concentration is increased in order to increase the print density, or when a design is made so that the pigment is remained on a surface of a recording medium without penetrating it, these problems are significant.
In order to improve such fixing properties of the pigment to the recorded matter, it has been proposed that a resin is added to the ink. This resin is considered to firmly fix the pigment onto the recorded matter as a binder. As such inks, there have been disclosed an ink containing a resin emulsion having a specific film-forming temperature (for example, patent documents 2 and 3) and an ink containing core-shell type resin particles each composed of a core portion and a shell portion enclosing it (for example, patent document 4).
Further, there has been disclosed an ink in which the amount of monomers contained in the polymeric dispersing agent and resin emulsion added to the ink in order to stably disperse the pigment is adjusted to 1,000 ppm or less (for example, patent document 5). Printing using this ink causes no irregular penetration to the recorded matter, so that an image of higher image quality can be stably obtained under diversified environments regardless of their environmental variation. Furthermore, unpleasant odors do not happen to occur during printing and after printing.
However, the ink for ink jet recording disclosed in this patent document 5 is an aqueous pigment ink in which the pigment is dispersed by using the polymeric dispersing agent. In such an ink, when the content of the pigment in the ink is increased in order to increase the print density of the printed matter, the ink viscosity also rapidly increases therewith in some cases. Further, the polymeric dispersing agent is excessively required to stably disperse the pigment in the ink, so that there has been the problem of insufficient ejection stability because of air bubble generation and a reduction in defoaming properties,
In addition, there has been disclosed an emulsified polymer in which the content of low-molecular weight components (oligomers) is specified to a specific amount or less (for example, patent document 6). By specifying the oligomer component content of the emulsified polymer to a specific amount or less, excellent durability and water resistance are realized when it is added to a paint or an adhesive. However, it is not suggested that this emulsified polymer is used as an additive to the ink for ink jet recording. Further, the upper limits of the specified molecular weight and content of the oligomers are relatively large. Accordingly, when it is applied to the ink for ink jet recording, there has been the possibility of causing defects such as unstable ejection.
Patent Document 1: JP-A-8-3498
Patent Document 2: JP-A-1-217088
Patent Document 3: JP-A-4-18462
Patent Document 4: JP-A-8-259869
Patent Document 5: JP-A-10-46073
Patent Document 6: JP-A-2003-96109